1. Field of the Invention
The present invention relates to arithmetic units for correcting a detection output from a detector in accordance with a linear input/output characteristic. More particularly, the present invention relates to an arithmetic unit, for correcting a detection output, in which a corrected operation output is sensitive to mechanical factors.
2. Description of the Related Art
There are various types of detectors which generate a detection output having a linear input/output characteristic. These detectors include a rotating-type or sliding-type variable resistor and a magnetic detector such as a Hall element.
An analog signal output generated by these detectors is converted by an A/D converter into a digital signal output, and the digital signal output is subjected to subsequent processing.
Since a digitally-converted value generated by the A/D converter may contain an error outside the allowable range, the digitally-converted value is generally corrected, and the corrected value is regarded as a detection output of the detector.
The corrected detection output value of the detector can be computed from the following equation (1) wherein the maximum detection output value for the allowable maximum input of the detector is stored as an upper limit value A, the minimum detection output value for the allowable minimum input is stored as a lower limit value B, and a straight line connecting the upper limit value A and the lower limit value B is approximating to a straight line representing the input/output characteristic of the detector:                               corrected          ⁢                                           ⁢          detection          ⁢                                           ⁢          output          ⁢                                           ⁢          value                =                                                            (                                  detection                  ⁢                                                                           ⁢                  output                                )                            -                              (                B                )                                                                    (                A                )                            -                              (                B                )                                              ×                      (                          2              n                        )                                              (        1        )            
wherein n is a natural number representing the number of bits of the A/D converter.
For example, when the A/D converter is an eight-bit converter, 2n (the number of discrete levels) is 256. When the A/D converter is a ten-bit converter, 2n is 1024.
When the eight-bit A/D converter is used, the values between the upper limit value A and the lower limit value B are allocated with a resolution of 256 (00 to FF). In accordance with this allocation, the detection output from the detector is converted into one of the values from 00 to FF.
When various types of detectors are influenced by mechanical or structural factors, such as when a rotating resistor is influenced by a mechanical angle error, by a mechanical error between a resistive element and a wiper, or by a change in resistance caused by a temperature change, the input/output characteristic (straight line) of the detector is apt to vary. Specifically, the actual input/output characteristic of the detector may increasingly deviate from the input/output characteristic based on the stored upper limit value A and the stored lower limit value B, and the corrected detection output value of the detector may include a large error. It has been confirmed that the average error range is approximately ±10%.